In delivering constant current electrical stimulation pulses by an implantable pulse generator to electrodes implanted near a stimulation site, a voltage converter (multiplier) is desirable to produce driving voltages from a power supply voltage. Providing a variable voltage for use in the foregoing has typically involved the use of an inductive voltage converter or a capacitive voltage converter.
An inductive voltage converter requires the use of a coil for voltage conversion, which in turn necessitates the use of alternating current. The use of such coils with alternating current often results in inefficiencies with respect to power consumption associated with voltage conversion. For example, where a battery is used as a power source, complicated and inefficient switching regulator circuitry is typically required to convert the direct current from the battery to alternating current for voltage conversion. Moreover, an inductive up-converter may introduce too much electronic noise to permit wireless (e.g., radio frequency) communication between an implantable pulse generator and an external control unit. This may require the inductive up-converter to be shut down periodically in order to “listen” for communication signals, resulting in output voltage droop.
Capacitive voltage converters avoid problems resulting from electronic noise, but typically have been limited to providing voltage output in integer multiples of a supply voltage (e.g., VBattery, 2VBattery, 3VBattery, etcetera). Such voltage converters only provide efficient operation at exact multiples of an input power supply voltage, with very poor efficiency at fractional multiples. In addition, depending on the design, capacitance for high voltage field effect transistors within the voltage multiplier may limit operating frequency. Such high voltage field effect transistors also consume substantial silicon area, while smaller field effect transistors lack acceptable reliability at high voltages that may be switched.